1. Field of the Invention
This invention relates to a dynamic optometer which is capable of measuring the refractive power of an eye from outside.
2. Description of the Prior Art
The conventional instruments for measuring the refractive power of eye are generally arranged to irradiate an eyeball with high-frequency pulses of infrared light which is converged into a beam, and a light spot which is formed on a retina as a secondary light source is imaged on a photo-detector to calculate the refractive power from the difference in position on the photo-detector.
However, the instruments of this sort require the to project infrared light from the front side of an eyeball. For this purpose, the direction of gaze of a subject has to be fixed in a straight forward direction, which causes considerable strain to the subject.
In order to lessen the strain on the part of the subject, the present inventors proposed in a prior co-pending application (Japanese Laid-Open Patent Application No. 61-52850) a dynamic optometer arranged to permit eye movements during measurements. This dynamic optometer employs an elliptic mirror to shift the apparent eye position, and a mirror which is rockable about two perpendicularly intersecting axes according to eye movements to establish an apparently fixed eye.
FIG. 4 diagrammatically illustrates the outline of the above-mentioned dynamic optometer, in which indicated at 1 is a light source/receiver which projects an infrared beam and receives the reflected light from the fundus to measure the refractive power, at 2 a flat mirror which is rockable about two axes, at 3 and 4 elliptic mirrors, and at 5 a subject's eyeball.
In this instance, when the eyeball 5 is in the direction indicated by the solid line, the flat mirror 2 is held in the position similarly indicated by the solid line by detection of the eyeball direction. Accordingly, the beam is projected into the eyeball 5 from its front side in the direction of gaze as indicated by arrows of the solid line. If the eyeball is turned to the direction indicated by the broken line, the flat mirror 2 is tilted into the position indicated by the broken line, projecting the beam into the eyeball 5 straight from the front side through the light path of the broken line in spite of the eye movement.
Although the elliptic mirrors 3 and 4 have a function equivalent to that of a lens, they have different lens characteristics at different positions on the elliptic surface due to variations in radius of curvature. Consequently, it becomes necessary to make corrections for the projection of the beam and measurement of reflected light by means of a computing circuit which is provided in the instrument.
Besides, there is another problem that the use of elliptic mirrors of high accuracy which are relatively difficult to machine is reflected by a considerable increase in cost of the instrument as a whole.